The present invention relates to a band-expanding method for optical amplifiers and to an optical amplifier. More particularly, the present invention relates to a band-expanding method for optical amplifiers, which expands the transmission optical signal band by adding on a pump laser source for an optically pumped amplification medium, such as an optical fiber, as well as to an optical amplifier used for the method. In particular, the method and the optical amplifier are used for optical fiber Raman amplifiers, optical repeaters, etc. employed for optical information communications.
The wavelength-division multiplexing (WDM) optical transmission method, that is used to transmit information by multiplexing a plurality of optical signals having different wavelengths from each other in an optical fiber, is very effective for increasing the capacity of optical fiber communication. To repeat/amplify an optical signal in such a way, however, it is required to use rare-earth doped optical fiber amplifiers, such as the EDFA (Erbrium-doped Fiber Amplifier), semiconductor optical amplifiers, and optical fiber Raman amplifiers, such as the optical fiber amplifier.
The WDM transmission apparatus combines those optical amplifiers so as to repetitively amplify each optical signal that has been attenuated due to the transmission thereof in an optical fiber as long as several tens of kilometers, thereby enabling the signal to be transmitted through a distance of more than several hundreds to several thousands of kilometers. Conventionally, the EDFA has been used as an optical amplifier of the commercial WDM transmission apparatus. As the WDM transmission apparatus is getting upgraded in band width and in capacity, the conventional EDFA amplification bands (about 30 nm, i.e. from 1530 nm to 1560 nm for conventional C-band optical amplifiers) have come to be short of transmission bands. In order to solve this problem, it is now under examination to use L-band EDFAs (about 30 nm, i.e. from 1570 nm to 1600 nm) having different wavelength bands from each other that are disposed in parallel and in which optical amplifiers are used to amplify the signal in each divided band, thereby making the transmission capacity double. Using of optical amplifiers in the 1300 nm band and in the S-band (1490 nm to 1520 nm) is also under examination for some transmission systems.
As described above, the WDM transmission apparatus often employs band-upgrading, that is, a band-expanding method that adds on optical amplifiers band by band so as to reduce the initial installation cost and cope with the transmission capacity requirement that is expected to arise in the future.
FIG. 2 shows a conventional method that uses a plurality of lumped optical amplifiers having different bands from each other as optical repeaters. An optical signal 101, after each wavelength thereof is multiplexed, is transmitted in an optical fiber transmission line 102-1, amplified in an optical repeater 108, and then output to an optical fiber transmission line 102-2. Because the number of signal wavelengths is small at the initial time, the method uses no band add-on optical amplifier 107 (dark portion). The optical signal is thus amplified only in a pre-installed optical amplifier 104. For example, a C-band EDFA or the like is usually used as the pre-installed optical amplifier 104. A wavelength band demultiplexer 105 and a wavelength band multiplexer 106 are disposed before and after the pre-installed optical amplifier 104, respectively, so as to cope with-bands to be added on in the future.
The above method enables both of the band add-on wavelength demultiplexer 105 and the wavelength band multiplexer 106 to be prepared beforehand, so that it is just required to provide a band add-on optical amplifier 107 in order to expand a band. As the band add-on optical amplifier, for example, any of L-band, S-band, and 1300 nm band optical amplifiers can be used. In the practical optical transmission, the configuration of any of pre-installed bands and add-on bands is arbitrary. For example, the C-band can be divided into sub-bands, each of which can be added on independently of others, or the L-band can be used as a pre-installed band.
In recent years, in addition to those optical amplifiers, it is also under examination to use the optical Raman amplifier (hereinafter to be referred to simply as the Raman amplifier) that uses an optical fiber as an amplification medium. The Raman amplification is a phenomenon in which an optical gain is caused to occur due to the Raman Effect from a pump wavelength within a range of about 110 nm at the longer wavelength side. The Raman effect is a kind of non-linear effect which is caused to appear when a strong pump laser beam (several tens of mW to several W) is entered into an optical fiber. The magnitude of the gain is not constant and a peak appears at a point spaced by about 100 nm from-the pump laser wavelength. It has a triangularly shaped gain characteristic in which the gain decreases smoothly up to the pump wavelength from the peak. Generally, the Raman amplification is employed for signal amplification, in which a Raman pump laser source is disposed in an optical repeater, an optical site, or the like so as to input the pump laser in the forward or backward direction in terms of the transmission direction of the signal light into the optical fiber transmission line, with use of the optical fiber transmission line itself as a gain medium. When a wavelength-multiplexed signal is amplified by such a Raman amplifier, a pump laser having a plurality of wavelengths is usually used, and the intensity of each pump laser is set to a proper ratio so as to flatten the gain within a wavelength range of the wavelength-multiplexed signal. It is well known that the method can obtain a flat gain up to about 100 nm.
On the other hand, while there is almost no case in which Raman amplifiers are used commercially, employment of the method is under examination. This is because the signal/noise (S/N) ratio of an optical signal can be improved and the transmission distance can be extended more than the case in which only the conventional lumped optical amplifiers are used when the Raman amplification and the lumped optical amplification are employed together. The U.S. Pat. No. 6,115,174 (document 1) discloses a method that uses the Raman gain to change signal powers and signal gain shapes.
The document 1 relates to setting of the amplification characteristic for each signal within a specific wavelength range. However, the document 1 does not mention the expanding of amplification bands and the adding-on of bands and pump laser sources so as to amplify signals in an add-on (newly-installed) wavelength range and a wavelength range (amplification range) of pre-installed amplifiers.
Unlike conventional lumped optical amplifiers, the gain band of the Raman amplifier cannot be added on band by band independently, since the optical gain of the pre-installed band and the add-on band is mutually overlapping. This is because the optical gain medium, that is, the optical fiber, is commonly used by all of the bands in the Raman amplifier.
It is well known that an increase in the number of wavelengths of Raman pump laser sources can improve the gain flatness and expand the gain bandwidth. A simple increase of pump laser sources, however, causes a problem in that the gain characteristic cannot be flattened nor shaped as desired after the add-on, since the optical gain bands-induced by the pre-installed pump-laser sources and the add-on pump laser sources are mutually overlapping. Hereinafter, this problem will be described in more detail.
FIGS. 3(a) to 3(c) are graphs which illustrate a synthesized gain of a Raman amplifier. FIG. 3(a) shows how both wavelength and intensity of pump lasers are adjusted so as to flatten the gain at 10 dB in a pre-installed C-band, where, for example, the gain ripple is less than 1 dB. On the contrary, FIG. 3(b) shows an example in which the wavelengths of the pump laser sources is shifted to the long wavelength side and set so as to flatten the gain in the L-band, as well. FIG. 3(c) shows the synthesized gain of FIG. 3(a) and FIG. 3(b), in which pump laser sources in FIG. 3(b) are simply added to the pump laser sources in FIG. 3(a).
FIG. 3(c) shows a simple example in which it is assumed that the gains are added linearly. Even in such a case, each of the gains in FIG. 3(a) and FIG. 3(b) is extended from a band into another band, so that the gains are synthesized and the result is shaped as shown in FIG. 3(c). As a result, the gain of the C-band, in which an optical signal is already transmitted, changes by about 10 dB at a maximum, thereby the optical signal might be degraded significantly. The gain band becomes uneven at the L-band side, so that the gain changes significantly at a wavelength of around 1570 nm and the signal transmission is disabled. It is well known that wavelength-multiplexed Raman pump lasers often interact each other due to the Raman effect in an actual Raman amplifier. Thus, gains are synthesized non-linearly and this makes it more difficult to add on pump lasers.
As described above, when a gain change occurs in a wavelength band in which an optical signal is already transmitted, the optical signal intensity changes significantly. The optical signal intensity increases so as to increase the non-linearity effect of the optical fiber, thereby the signal transmission property is degraded. Some of the wavelength-multiplexed signals are extremely amplified, so that the gains of other signals are reduced and the S/N ratio of each signal is degraded. It is also expected that the signal intensity change causes the allowable input range of a receiver to be exceeded, thereby the receiver is damaged, or the receiver automatic-gain control (AGC) circuit is unable to track an abrupt power change at a pump laser add-on time, resulting in transmission errors. Similar problems might also arise even in a new add-on band, since the signal gain cannot be flattened after the add-on.
For example, the logical threshold level in a receiver is usually set to around 50%. Consequently, when the optical signal gain drops abruptly by 3 dB at an add-on time, the amplitude of the received signal in the receiver drops by 50%, thereby the signal amplitude in the receiver further goes down, resulting in a problem in that the signal reception is disabled completely. This is why the gain change in the first band must be suppressed at the highest within 2 dB.
Under such circumstances, it is an object of the present invention to provide an optical amplifier that has solved the above-described conventional problems. Concretely, it is an object of the present invention to provide a band-expanding method for optical amplifiers, which can prevent optical signals in the pre-installed bands from degradation and the add on of new wavelength bands while the gains of the pre-installed bands are held as they are.
It is another object of the present invention to provide a wide band optical amplifier that can share a single gain medium with a plurality of wavelength bands used for the method, and an apparatus to be used for the optical amplifier.
In order to achieve the above objects, the band-expanding method of the present invention for optical amplifiers expands an amplification band for optical amplifiers so as to amplify an optical signal in a second transmission band at a predetermined gain characteristic with use of an optical amplifier having a first pump laser source used to amplify an optical signal in a first transmission band, which is different from the second transmission band.
The band-expanding method of the present invention comprises: a first step of adding on a second pump laser source separately from the first pump laser source, and a second step of controlling both of the first and second pump laser sources after the processing in the first step so that the first and second transmission bands have a predetermined gain characteristic, respectively.
The first pump laser source need not necessarily be a single pump laser, rather, it may be a plurality of laser sources. Similarly, the second pump laser source also may be a plurality of laser sources. The second pump laser source is added on in two ways; in one way, a second pump laser source is connected physically, and in the other way, a pump laser source that is not turned on is prepared beforehand, then it is turned on (activated).
In a preferred embodiment of the present invention, the practical property of each optical amplifier can be improved especially when the distributed Raman amplifier is used together with a lumped optical amplifier, such as a rare-earth doped optical fiber amplifier and a semiconductor amplifier. In another preferred embodiment of the present invention, the output intensity values of some or all of the pre-installed (the first pump laser sources) and the add-on pump laser sources (the second pump laser sources) are changed simultaneously when pump laser sources to be added on are turned on in the second step so as to prevent transmission signals from degradation in the add-on transition state of the second pump laser sources. Specifically, the degradation of the optical signal in the pre-installed band caused by the change of the signal gain and the signal power during the band add-on process can be suppressed as follows. The pump powers are carefully controlled at each instance from the start state through the transition state to the end state, so that the total optical gain characteristic of the pre-installed band and the add-on band matches the specified gain profile; for example, the gain of the pre-installed band is unchanged through all the states from the start to the end.
The method for setting the pump laser intensity in a transition state can be realized easily, for example, when the ratio between output intensities before and after the add-on or the ratio of each wavelength to the pump laser output intensity is changed approximately linearly with respect to time so as to shift the state before the add-on to the state after the add-on. This is also effective even when pump lasers interact with each other to some degree.
Furthermore, in order to achieve the above objects, the optical amplifier of the present invention includes a Raman amplifier having a first pump laser source used to amplify an optical signal in a first transmission band (an amplification band before the add-on) and means for adding on a second pump laser source which is used to amplify an optical signal in a second transmission band (amplification band after the add-on) with use of the above Raman amplifier. The second transmission band is different from the first transmission band. The optical amplifier is configured controllably so as to obtain the gains of the first and second transmission bands described above as predetermined gain characteristics.
The means for adding on the above-mentioned second pump laser includes a state in which the second pump laser source is connected to a distributed Raman amplifier and is inactive electrically. The means also includes setting means for controlling either the wavelength or the intensity of the above first and second pump laser sources.
The present invention is especially effective in a distributed optical amplifier that is used as a means for amplifying optical signals. The distributed optical amplifier employs the distributed Raman amplification that uses an optical fiber transmission line as an amplification medium. The present invention can prevent optical signals in pre-installed bands from undergoing intensity changes by setting the amplification gain or intensity of each optical signal in pre-installed bands to an approximately fixed value before and after the add-on of a pump laser source. Consequently, it is possible to prevent degradation in the transmission of optical signals caused by the optical fiber non-linearity effect and SIN degradation, as well as preventing deviation of the input ranges, damage, etc to the receivers. For example, it is possible to prevent the items from significant degradation by suppressing gain/light intensity changes under 2 dB.
The output intensity values, as well as the ratio of output intensity values of those pre-installed and add-on pump laser sources before, after the add-on, or in an add-on transition state may be stored as electronic information or reference voltages in the apparatus or obtained from the outside with use of a monitor signal, header information, etc. They may also be written as electronic information when each of the above pump laser sources is added on or the apparatus may be configured so as to receive those information items from the reference voltage source or the memory in the add-on module.
Furthermore, the apparatus may be configured so as to have a variable gain equalizer for controlling the gain or the gain shape of each of the wavelength bands in which pre-installed optical signals are transmitted in order to hold the gain or intensity of each pre-installed optical signal fixedly before, after the add-on or in an add-on transition state.
Other and further objects, features and advantages of the invention will appear more fully from the following description.